Plate graining apparatus

ABSTRACT

An apparatus adapted to grain a planar surface of a plate comprising a moving conveyor which carries such plate into and out of contact with moving graining means which operate to impress grit into contact with the planar surface and produce a random roughening thereof.

Hathaway Mar. M, 11972 [54] PLATE GRAINING APPARATUS [56] ReferencesCited [72] Inventor: Charles T. Hathaway, 518 Rancho Lindo UNITED STATESPATENTS ri i Calif- 91722 I 899,236 9/1908 Botz ..51/136 [22] Filed:Sept 23, 1970 2,739,426 3/1956 Kinker et a1. ..51/138 [21] App1.No.:74,700 Primary Examiner-0thell M. Simpson Attorney-Huebner & Worrell[52] U.S.Cl ..5l/61,51/136,51/328 [57] ABSTRACT [51] Int. Cl. ..B24b7/12, B24b 21/04, B24b 1/00 An apparatus adapted to grain a planarSurface ofa plate [58] Field of Search ..51/135-147, 328, prising amoving conveyor i h carries such plate into and out of contact withmoving graining means which operate to impress grit into contact withthe planar surface and produce a random roughening thereof.

11 Claims, 10 Drawing Figures PLATE GRAINING APPARATUS BACKGROUND OF THEINVENTION This invention is particularly adapted to the preparation ofaluminum lithographic plates, which are in almost universal use forcommercial printing runs of 5,000 to 10,000 copies or more. Regardlessof whether stone, zinc, aluminum, paper or plastic plates are used thelithographic process depends on establishing chemically contrastingareas: hydrophilic or water-loving," ink repellent nonprinting areas andoleophilic or ink-loving, ink receptive, printing areas. Raw aluminumgenerally has or will develop an oleophilic surface so that is is firstnecessary to provide a completely hydrophilic surface upon the plate bychemical and/or mechanical means. In practice the plate is then madelight sensative and portions of this hydrophilic area are selectivelyconverted to oleophilic areas by exposure to light through aphotographic negative (or positive) transparency.

This invention is concerned with the essential first stage of convertinglarge area plates from erratically performing surfaces to a very uniformhydrophilic surface.

There are three fundamental existing graining processes, the chemicaletching process, the ball graining process and the brush grainingprocess. In the chemical process plates are etched under mild alkalineconditions. The etching removes about 0.0001 to 0.001 inches of thesurface eliminating all surface soil and oxidation, exposing freshmetal. The ball graining process was developed for zinc plates in thelate 1800s and in the early 1960s was found to be applicable to aluminumas well as to zinc plates. This is a physical operation, involving therotation of steel, glass, or even wood marbles over the surface of theplate in a graining machine. Water is added and then an abrasive gritmaterial. The actual roughening of the surface of the metal to produce agrain is done by the abrasive grit.

In the brush graining method the plates are passed under rotatingbrushes and a water slurry of crushed silica to produce a mechanicallycleaned and roughened surface.

The chemical etching process reduces the gauge or thickness of the metalslightly, requires numerous chemical baths, and produces an essentiallysmooth plate which is normally more difficult to control on the printingpress and is usually of somewhat shorter run (number of impressionsobtainable). The etching process requires a substantial investment inequipment to move the plate from one bath to another, or an even greaterinvestment in equipment to carry a continuous web through a series ofchemical baths, rinsing the plates between each bath to avoid carryoverfrom bath to bath. It is susceptible to a great deal of handling lossdue to mechanical damage of the plate and due to the total loss ofsubstantial numbers of plates any time the process is interrupted forany reason. This is due to the fact that anywhere from a half an hour toan hour and a half or 2 hours of production material is tied up or inprogress at any given time with this system so that any interruption ofthe process can result in substantial loss of material which at bestmust be reprocessed and frequently must be totally discarded.

The ball graining method suffers the disadvantage of extremely highlabor costs due to the long cycle time and the essentially handoperation of loading the tables, dumping the balls from the tables,cleaning the plates, washing the table and reloading with fresh plates.Although the equipment costs are relatively low for an individual table,many tables are required to produce any significant amount ofproduction.

Brush graining is probably the most economical method of production inoperation today but suffers a significant disadvantage in that it cannotproduce the depth of grain that ball graining can. Furthermore, controlof the process is extremely subjective in that the bristles of the brushare constantly being worn in a manner which is difficult to measure orevaluate. The pressure of the bristles against the plate is difficult tocontrol and the distribution of the grit across the plate is alsodifficult to regulate. In actual practice, it is frequently found thatseveral hours of production time may be lost while an effort is made tofind out what combination of brush setting, grit composition, flowrates, r.p.rn., and so on will produce the desired grain. Also, standingwaves or vibrations frequently form in the brushes which result inpatterns on the plate surfaces and do not produce a uniform hydrophilicfinish. The system does have the advantage that it can be and has beenadapted to work with coil operations.

SUMMARY OF THE INVENTION The invention preferably includes an endlessconveyor upon which untreated or ungrained plates are randomlypositioned. A series of graining means such as moving chains, belts, orthe like are positioned across the conveyor and normal to the line oftravel of the conveyor. As the plates to be grained are moved along theconveyor belt they come in contact with the chains. At the same time, agrit slurry consisting of finely crushed silica or quartz or the like ina liquid suspension is sprayed onto the plate surfaces. The movingchains, belts or the like rub the grit slurry against the plate surfacesimparting a graining finish to the surfaces.

It is an object of this invention to provide apparatus and a method forgraining surfaces which is simpler to operate and easier to control thanexisting graining methods.

It is a further object of this invention to provide apparatus and agraining method which is controllable to produce any desired graindepth, from the very shallow 2 to 5 micron peak to valley grain depth ofthe brush graining method up to the 10 to 15 micron peak to valley depthof the ball graining method.

The present invention, unlike brush and ball graining methods can beused with any gauge plate.

An additional purpose of the present invention is to provide a grainingmethod which is both more efiicient and less expensive than currentgraining methods.

As with any of the currently available graining processes, thisinvention relates only to the first step in the preparation oflithographic plates. Further, chemical "treatment of the plates isessential to provide a long lasting hydrophilic surface. However, thesechemical treatments affect the surface on a molecular scale, e.g., 1-50Angstroms, whereas the treatment of interest in this invention relatesto physical structure on a microscopic scale, e.g., 1-10 microns. Thesechemical treatments are ineffective without exposure of clean freshlygrained plate surfaces.

Other objects and advantages will become apparent from the followingdrawings and description, wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. l is a top elevational view ofthe graining apparatus of this invention;

FIG. 2 is a cross sectional view of the graining apparatus taken alongline 2-2 of FIG. 1;

FIG. 3 is a fragmentary, partial cross sectional view of the apparatustaken along line 3-3 of FIG. 1;

FIG. 4 is an enlarged cross sectional view of one graining chain takenalong line 4-4 of FIG. 3;

FIG. 5 is a view similar to FIG. 3 illustrating a modification of theinvention.

FIG. 6 is a view similar to FIG. 3 and illustrates a modificationwherein V-belts are substituted for the graining chains;

FIG. 7 is an enlarged cross sectional view of a V-belt taken along line7-7 of FIG. 6;

FIG. 8 is a top elevational view of a further modification of theinvention;

FIG. 9 is a partial cross sectional view taken along line 99 of FIG. 8;and

FIG. 10 is a top elevational view of still a further modification of theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1 there isillustrated the graining apparatus of the invention generally designated12. The apparatus 12 comprises a frame 14 which supports an endlessconveyor belt 16. Roller 18 is one of a series of rollers which provideadditional support the the belt 16 and which also drive the belt 16. Oneor more of the rollers 18 may be powered by any suitable motor means(not illustrated) so as to move the conveyor belt 16 in the directionindicated by the large arrow in FIG. 1. Mounted on vertical supportposts or frame extensions 20 and 22 of frame 14 is a grit dispersingmeans or hollow slurry pipe 24, which extends laterally across theconveyor belt 16 at a height of ap proximately 1 foot above the belt 16in the preferred embodiment. A hose 26 is connected to slurry pipe orconduit 24 by means of a coupling 28. The hose 26 connects the slurrypipe 24 with a slurry storage tank, not shown. While the structure justdefined is preferred, any ,type of appropriate apparatus may be used todistribute a grit slurry.

Horizontal support brackets 30 and 32 extend outwardly from frame 14.These brackets 30 and 32 are at approximately the same height or levelas the conveyor belt 16. An electric motor 34 is mounted on bracket 32.A drive shaft 36 extends from motor 34 and is joumaled in a bearingmount 38. A gear wheel 40 is mounted on drive shaft 36 near motor 34. Adrive shaft 42 extends between and is supported by two bearing mounts 44and 46 respectively. Shaft 42 lies parallel to drive shaft 36. A gearwheel 48 is mounted on shaft 42 and meshes with gear wheel 40.Therefore, when motor 34 is energized and turning shaft 36 in aclockwise direction, the meshing of gears 40 and 48 will cause shaft 42to rotate in a counterclockwise direction. In other words, the shafts 36and 42 will always rotate in opposite directions.

A pair of parallel, free turning shafts 50 and 52 are positioned uponbracket 30 and mounted in bearing mounts 54, 56, 58 and 60,respectively. Pulley wheels 62 are mounted on shafts 36 and 42 and areaffixed thereon by any appropriate means such as by set screws or welds.Idler pulleys 64 are mounted on shafts 50 and 52 and may be affixedthereto or may be free to rotate independently of the shafts 50 and 52.

Graining means or elongated grit engaging elements generally designated65, such as an endless chain 66 formed of articulated links 67 aremounted around pulleys 62 and 64 as can best be seen in FIG. 1 so as toextend horizontally across conveyor belt 16.

Thus, when the drive shaft 36 of motor 34 rotates in a clockwisedirection those chains 66 mounted on the pulleys 62 of drive shaft 36and the pulleys 64 of drive shaft 50 will also rotate in a clockwisedirection. Concurrently, those chains 66 mounted on the pulleys 62 and64 affixed to shafts 42 and 52 respectively, will be rotated in acounterclockwise direction.

In operation the plates 68 to be grained are placed upon the conveyorbelt 16 near roller 18 either randomly or at predetermined intervals byhand or by a machine (not shown). As the plates 68 pass under slurrypipe 24 the slurry mixture 69 of crushed silica and water is sprayedonto plates 68 from nozzles or apertures 70 located at intervals alongthe bottom of pipe 24. The plates 68, with the slurry mixture spreadacross their upper surfaces, are then passed underneath the lowersections 72 of chains 66. As the chains 66 are drawn across the surfacesof plates 68 the silica grit particles 73 are forced into the relativelysoft metal so as to impart the desired grained or roughened finish tothe plate surfaces 68. The grained plates 68 then pass through a seriesof wash baths and chemical treatment baths (not shown) which completethe process of imparting a uniform hydrophilic surface to the plates 68.

In the preferred embodiment illustrated in FIG. 1 through FIG. 4,pulleys 62 and 64 all lie in substantially vertical planes, and theendless chains 66 thus have substantially vertical orbits. It isenvisioned, however, that the pulleys 62 and 64 could be mounted in ahorizontal plane parallel to and slightly above the plane of theconveyor belt 16. Chains 66 would then rotate in a horizontal orbit andboth sides of the chains 66 would contact plates 68 and belt 16.

Additionally, while the grit engaging elements 65, as illustrated,extend substantially laterally across conveyor belt 16 in a directionnormal to the line of travel of conveyor 16, it is anticipated that thegrit engaging elements 65 could be placed so that they extend parallelto the line of travel of conveyor 16. This could be accomplished bysuspending the means imparting a horizontal shifting of the chains 66,that is, the motor pulley and shaft assemblies, above the conveyor.

The frame 14 includes gutters or drain channels 74 and 76 along bothsides of the conveyor belt 16 where excess grit slurry 69 can collect.These channels 74 and 76 may be angled downwardly slightly from one endof the frame 14 to the other end thereof to facilitate draining of thegrit slurry 69 so that it may be recycled. It should be understood thatmore than one slurry pipe 24 may be used and that such slurry pipes 24may be positioned directly over the endless chains 66.

It should also be understood that the embodiment illustrated in FIG. 1,which includes four chains, two of which rotate in a clockwise directionand two of which rotate in a counterclockwise direction, is so shown forpurposes of clarity and simplicity. Additional chains may be usedwithout departing from the spirit of the invention. It is believed thata moderate number of chains, 15 for example, which can be driven atspeeds well in excess of feet per minute, can completely grain orroughen plates on a conveyor belt moving at rates in the range of l to10 feet per minute. The chains travel in alternate clockwise andcounterclockwise directions so that the plates 68 and the grit slurry 69will not all be pulled to the same side of the conveyor belt 16 and toinsure a random roughening.

There is a suflicient degree of slackness in the chain sections 72 sothat the individual chain links 67 are free to adjust to the inherentthickness and contour variations of the plates being grained. Thisfreedom to adjust to various contour and gauge variations means that theinvention is capable of handling different gauge plates at the same timeand without elaborate or time consuming equipment modifications.

There are several factors which may influence the depth of the grainimparted by the apparatus of this invention. These factors include gritparticle size, conveyor belt speed, chain loop speed, chain link weight,and chain wear. Referring to the latter factor, after the chains havebeen in use for a period of time the edges of the chain links 67 willwear down or flatten somewhat, as seen in FIG. 4, thereby creating moresurface area to press the grit particles 73 into the plate surface.Recent tests have indicated that the process is substantially lesssensitive to variations in chain link mass or grit particle size thanoriginally expected. It now appears that the principal variable isactually the hardness of the chain link metal itself. A hard steel chainwill give grain depths of 6 to 8 microns with either fine or coarse gritand with either heavy chain or light chain while a soft surface such asrubber or leather will give a depth of 4 microns over the same broadrange of chain weight and grit size.

In FIG. 5 there is illustrated a modification of the invention whereinthe grit engaging elements or chains 66 include a rubber hose 79 whichsurrounds the chains 66'. Because the rubber hose 79 is softer than thechains 66', the grit particles 73 are not pressed into the plate 68 withas much force or as deeply as is done by the chain links 67 of FIGS. 1through 4, resulting in a shallower grain.

FIGS. 6 and 7 illustrate a further modification of the grit engagingelements 65 of the invention wherein these elements comprise rubberV-belts 66". As seen in FIG. 7, the V-belts 66" are frustoconical incross section. As with the modification shown in FIG. 5, the V-belts 66"are relatively soft and do not press the grit particles 73 into theplate 68 with as much force as do the chains 66, thereby imparting tothe plate 68 a shallower grain than would normally be obtained by usingexposed chains 66 as the grain effectuating elements.

FIG. d illustrates a further modification of the invention wherein thegrit engaging elements 65' comprise cylindrical graining bars 80. Motor34 mounted on frame bracket 32 is connected to an offset crank shaft 82.Connecting bars M, which are mounted on and free to rotate about theoffset subshafts 86 of crank shaft 82, are connected to bars 80 at oneend thereof by means of pivot pins 99. Bars M lie laterally acrossconveyor 16. The opposite ends 90 of bars 80 are reduced slightly indiameter and extend into sleeves 92, which are mounted on L-frame member94 of frame bracket 30. Helical springs 96 may be coupled to the reduceddiameter end portion 90 of graining bars $0 and to sleeves 92 by meansof spot welds or other appropriate methods. These springs operate tourge or draw graining bars 90 away from crank shaft 82 and toward framemember 94. In operation plates 68 placed upon conveyor belt 116 passunder graining bars Ml. As motor 34 rotates, crank shaft b2 and the bars80 move laterally and reciprocally normal to the direction of travel ofbelt 16 pressing grit particles 73 into plates 68.

in FIG. 110, a drive wheel 98 of motor 34 is connected to reciprocatingbar 100 by means of coupling bar 1102. Reciprocating bar 1100 issupported in slide bearings 11M mounted on frame bracket 32. Stationaryframe member 94' is mounted parallel to reciprocating bar 100 and framebracket 30. Both reciprocating bar we and stationary frame member 94include a series of eyes or hooks lilo. Grit engaging elements 65"include chains 108 which are hooked or otherwise fastened to the eyes106 of reciprocating bar 100 and frame member 94' so that chains 108 lielaterally across conveyor belt 16. When motor 34 is activated so as tomove reciprocating bar 100 back and forth in a line parallel to thedirection of travel of conveyor belt 16, chains 1108 move or whipeccentrically upon conveyor belt 16. As with the other modifications ofthe graining means 65 the chain links 67' operate to press gritparticles 73 into plates 68 thereby graining the plates.

After the roughening of plates 68, the residue of grit 69 is removed byconventional means and the plate is then ready for further conventionalprocessing to prepare it for lithographic use.

While the invention described herein is primarily intended for use inthe lithographic process, it should be understood that the process isapplicable to the treatment of other surfaces where a roughened orgrained appearance is desired, as with frosted glass, light diffusers,matte finished plastic drafting media, and burnished decorative metalpanels.

Although I have herein shown and described my invention in what l haveconceived to be the most practical and preferred embodiment it isrecognized that departures may be made therefrom within the scope of myinvention.

What is claimed is:

1. An apparatus adapted to roughen a planar surface of a plate wherein aroughing agent in the form of a grit is used in cooperation with saidapparatus comprising:

a conveyor supporting frame assembly;

an endless conveyor belt supported by said assembly in a mannerpresenting a generally horizontal portion adapted to receive and movesaid plate;

motor means associated with said belt adapted to drive said belt;

dispersing means adapted to disperse grit on said planar surface duringtravel of said plate on said belt; and

a plurality of elongated grit engaging elements overlying said generallyhorizontal portion of said conveyor belt and said planar surface, saidelements possessing sufficient weight to gravitationally bear upon thegrit deposited on said planar surface;

means imparting a horizontal shifting of said elements while in contactwith such grit to impress the grit against the planar surface andproduce a random roughening thereof.

2. An apparatus for graining planar surface as defined in claim 1wherein:

said dispersing means comprises a tubular conduit mounted on said frameassembly so as to extend laterally across said conveyor belt, and saidconduit includes apertures spaced therealong to release said grit ontosaid planar surface.

3. An apparatus for graining planar surfaces as defined in claim 1wherein:

said conveyor supporting frame assembly includes at least one parallelgutter disposed along said frame assembly adapted to receive excess gritduring the roughening of said planar surface.

d. An apparatus adapted to roughen a planar surface as defined in claimll wherein:

said elongated grit engaging elements comprise an endless chain ofarticulated chain links.

5. An apparatus adapted to roughen a planar surface as defined in claim4 wherein:

said endless chain is enclosed in a rubber tubing.

6. An apparatus adapted to roughen a planar surface as defined in claim1 wherein:

said elongated grit engaging elements are rubber flexible,

resilient belts. 7. An apparatus adapted to roughen a planar surface asdefined in claim ll wherein:

said grit engaging elements include cylindrical bars which extendlaterally across said conveyor belt and said means imparting ahorizontal shift to said elements comprising a motor driven offsetcrankshaft imparting alternately reciprocating lateral movement of saidbars across said conveyor belt. d. An apparatus adapted to roughen aplanar surface as defined in claim ll wherein:

said grit engaging elements include articulated chain lengths and saidhorizontal shifting means comprises a motor driven reciprocating barmounted on said frame assembly substantially parallel to the line oftravel of said conveyor belt and a stationary bar mounted on said frameassembly laterally opposite and parallel to said reciprocating bar;

one end of said chain lengths being fastened to said reciprocating barand the other end of said chain lengths being fastened to saidstationary bar, whereby when said reciprocating bar is activated saidchain lengths horizontally whip.

9. An apparatus adapted to roughen a planar surface of a plate wherein aroughing agent in the: form of a grit slurry is used in cooperation withsaid apparatus comprising:

a conveyor supporting frame assembly;

an endless conveyor belt supported by said assembly in a mannerpresenting a generally horizontal portion adapted to receive and movesaid plate;

motor means associated with said belt adapted to drive said belt;

dispersing means adapted to disperse said grit slurry on said planarsurface during travel of said plate on said belt; and

a plurality of endless chains formed of articulated links lying normalto the line of travel of said conveyor and overlying said generallyhorizontal portion of said conveyor belt and said planar surface, saidchains representing a section possessing sufficient weight togravitationally bear upon the grit slurry deposited on said planarsurface;

drive means adapted to rotate said chains imparting a horizontalshifting thereof while in contact with such grit slurry to impress thegrit slurry against the planar surface and produce a random rougheningthereof.

10. An apparatus as defined in claim 9 wherein:

the planar surfaces of a plurality of plates may be simultaneouslygrained.

1111. The method of graining the planar surface of a plate adapted to befurther processed to form a complete lithographic plate including thesteps of:

selecting a metal plate of appropriate hardness and size having agenerally flat planar surface;

positioning said plate on a moving conveyor with said planar surfaceexposed upwardly;

such grit slurry to impress the grit within said grit slurry against theplanar surface and produce a random roughening thereof, and removingsaid grit slurry from said plate t l l I t

1. An apparatus adapted to roughen a planar surface of a plate wherein aroughing agent in the form of a grit is used in cooperation with saidapparatus comprising: a conveyor supporting frame assembly; an endlessconveyor belt supported by said assembly in a manner presenting agenerally horizontal portion adapted to receive and move said plate;motor means associated with said belt adapted to drive said belt;dispersing means adapted to disperse grit on said planar surface duringtravel of said plate on said belt; and a plurality of elongated gritengaging elements overlying said generally horizontal portion of saidconveyor belt and said planar surface, said elements possessingsufficient weight to gravitationally bear upon the grit deposited onsaid planar surface; means imparting a horizontal shifting of saidelements while in contact with such grit to impress the grit against theplanar surface and produce a random roughening thereof.
 2. An apparatusfor graining planar surface as defined in claim 1 wherein: saiddispersing means comprises a tubular conduit mounted on said frameassembly so as to extend laterally across said conveyor belt, and saidconduit includes apertures spaced therealong to release said grit ontosaid planar surface.
 3. An apparatus for graining planar surfaces asdefined in claim 1 wherein: said conveyor supporting frame assemblyincludes at least one parallel gutter disposed along said frame assemblyadapted to receive excess grit during the roughening of said planarsurface.
 4. An apparatus adapted to roughen a planar surface as definedin claim 1 wherein: said elongated grit engaging elements comprise anendless chain of articulated chain links.
 5. An apparatus adapted toroughen a planar surface as defined in claim 4 wherein: said endlesschain is enclosed in a rubber tubing.
 6. An apparatus adapted to roughena planar surface as defined in claim 1 wherein: said elongated gritengaging elements are rubber flexible, resilient belts.
 7. An apparatusadapted to roughen a planar surface as defined in claim 1 wherein: saidgrit engaging elements include cylindrical bars which extend laterallyacross said conveyor belt and said means imparting a horizontal shift tosaid elements comprising a motor driven offset crankshaft impartingalternately reciprocating lateral movement of said bars across saidconveyor belt.
 8. An apparatus adapted to roughen a planar surface asdefined in claim 1 wherein: said grit engaging elements includearticulated chain lengths and said horizontal shifting means comprises amotor driven reciprocating bar mounted on said frame assemblysubstantially parallel to the line of travel of said conveyor belt and astationary bar mounted on said frame assembly laterally opposite andparallel to said reciprocating bar; one end of said chain lengths beingfastened to said reciprocating bar and the other end of said chainlengths being fastened to said stationary bar, whereby when saidreciprocating bar is activated said chain lengths horizontally whip. 9.An apparatus adapted to roughen a planar surface of a plate wherein aroughing agent in the form of a grit slurry is used in cooperation withsaid apparatus comprising: a conveyor supporting frame assembly; anendless conveyor belt supported by said assembly in a manner presentinga generally horizontal portion adapted to receive and move said plate;motor means associated with said belt adapted to drive said belt;dispersing means adapted to disperse said grit slurry on said planarsurface during travel of said plate on said belt; and a plurality ofendless chains formed of articulated links lying normal to the line oftravel of said conveyor and overlying said generally horizontal portiOnof said conveyor belt and said planar surface, said chains representinga section possessing sufficient weight to gravitationally bear upon thegrit slurry deposited on said planar surface; drive means adapted torotate said chains imparting a horizontal shifting thereof while incontact with such grit slurry to impress the grit slurry against theplanar surface and produce a random roughening thereof.
 10. An apparatusas defined in claim 9 wherein: the planar surfaces of a plurality ofplates may be simultaneously grained.
 11. The method of graining theplanar surface of a plate adapted to be further processed to form acomplete lithographic plate including the steps of: selecting a metalplate of appropriate hardness and size having a generally flat planarsurface; positioning said plate on a moving conveyor with said planarsurface exposed upwardly; spreading a grit slurry over said entireplanar surface; activating elongated grit engaging elements overlyingsaid conveyor and said planar surface, said elements possessingsufficient weight to gravitationally bear upon the grit slurry spread onsaid entire planar surface to impart a horizontal shifting of saidelements while in contact with such grit slurry to impress the gritwithin said grit slurry against the planar surface and produce a randomroughening thereof, and removing said grit slurry from said plate.